Despite marked improvements in dialysis technology and emphasis on delivery of an adequate dose of hemodialysis, a majority of the 300,000 patients undergoing conventional thrice-weekly hemodialysis (CHD) have sleep apnea, poor sleep quality, and daytime symptoms of sleepiness and fatigue. Since hemodialysis patients report symptoms of daytime sleepiness and fatigue would strongly influence their choice of dialysis modality and many hemodialysis patients feel impaired by sleepiness and fatigue, it is crucial to characterize how nocturnal hemodialysis will impact sleep. In light of the findings from the MEMO Study which demonstrated no significant quality of life or survival benefit from increased dialysis dose or membrane flux, attention has turned to the potential benefits of nocturnal hemodialysis (NHD). NHD provides the highest dose of dialysis and is more likely to decrease physiologic variations over time compared to any other type of hemodialysis. In small, uncontrolled studies, NHD has been reported to benefit daytime symptoms and reduce sleep apnea. The NI DDK-sponsored Frequent Hemodialysis Network (FHN) is a randomized clinical trial to determine whether NHD (6 times per week) leads to significant improvements in cardiovascular function and physical well-being compared to CHD over a 12-month period. We propose to complement this study with measurements of sleep and fatigue which utilize portable sleep apnea monitors, naturalistic measures of sleep-wake behavior, frequent ecologic sampling methods of sleepiness and fatigue, and centralized telephone surveys of sleep quality in order to gather crucial information of sleep and fatigue while limiting the burden on the parent study. Specifically, we propose to determine the impact of nocturnal hemodialysis on sleep apnea, to evaluate the effect of nocturnal hemodialysis on sleep quality and sleep/wake behaviors, and to assess the benefit of nocturnal hemodialysis on daytime symptoms of sleepiness and fatigue compared to conventional hemodialysis. The proposed study takes advantage of a multi-disciplinary team which offers an exceptional foundation to assess sleep and fatigue among those undergoing hemodialysis in the FHN Trial. This proposal addresses a major public health concern since the CHD population has been shown to have high rates of sleep disorders and reports severe daytime symptoms of sleepiness and fatigue.